A control device of a brushless DC motor (hereinafter abbreviated as a “motor”) used for a blower motor or the like of a vehicle air conditioner causes an inverter circuit to generate a voltage having a duty ratio corresponding to a target rotation speed, and applies the generated voltage to a coil of the motor.
An inverter circuit 40 includes a switching element. The inverter circuit 40 generates a voltage to be applied to the coil of the motor by a pulse width modulation (PWM) that adjusts a voltage by turning on and off the switching element to modulate a DC voltage of a power supply into a pulsed waveform. However, noise may be generated from the motor according to the PWM control.
In order to suppress the noise caused by the PWM, spread spectrum that sequentially changes one cycle of the waveform of the voltage generated by the PWM is effective. In order to change one cycle of the waveform of the voltage, a carrier frequency related to the generation of the PWM signal for turning on and off the switching element is changed. By changing the carrier frequency, the noise from the motor due to the PWM varies every short time and uniformly diffuses, such that the noise from the motor is reduced.
Even when one cycle of the waveform of the voltage changes, if a duty ratio that is a ratio of the time of one pulse generated by turning on the switching element to one cycle of the waveform of the voltage is the same, an effective value of the voltage applied to the coil of the motor does not change in theory. FIG. 8A shows a case in which one period of a PWM is long, and FIG. 8B shows a case in which one period of a PWM is short. However, since the duty ratios are all X %, the effective value of the voltage applied to the coil of the motor is theoretically the same.
Japanese Patent Application Laid-Open (JP-A) No. 2013-62933 discloses an invention of an electric compressor in which, when a PWM signal is generated from a signal wave and a carrier wave, a carrier frequency of the PWM signal is randomly switched by spread spectrum.